Conventional metal or compound superconductors that are used as Josephson devices of a tunnel junction include those which have niobium pentoxide (Nb.sub.2 O.sub.5), silicon (Si) or aluminum oxide (Al.sub.2 O.sub.3) connected as an insulating layer between two niobium (Nb) layers, as well as those which have amorphous silicon or magnesium oxide (MgO) connected as an insulating layer between two niobium nitride (NbN) layers.
Since the oxide high-temperature superconductor has a short coherence length, the fabrication of tunnel junction type Josephson devices requires providing a thin (&lt;100 nm) and uniform insulating layer. Such insulating layers can be prepared fairly easily by oxidizing the surface of metals, thus currently fabricated Josephson devices of the tunnel junction type have and insulating metal oxide layer sandwiched between an oxide high-temperature superconductor and a metal superconductor.
Tunnel junction type Josephson devices having an insulating layer sandwiched between oxide high-temperature superconductors are also available and one example is a device that has a barrier PrBa.sub.2 Cu.sub.3 O.sub.x sandwiched between two YBa.sub.2 Cu.sub.3 O.sub.x layers. In this device, PrBa.sub.2 Cu.sub.3 O.sub.x forming the barrier layer is in a rhombic system that is a different crystal system than the superconducting phase and, hence, nonuniformity of the critical current density along the barrier layer may result. To solve this problem, a Josephson device has been fabricated in which niobium (Nb) doped SrTiO.sub.3 is sandwiched between YBaCuO superconductors.
The Josephson devices described above that use oxide high-temperature superconductors are fabricated by epitaxial growth, so the combinations of materials from which the devices can be manufactured are very limited from the viewpoint of matching between the superconductor and the barrier layer in terms of electrical and crystallographic characteristics.
It is known that an irradiated part of an oxide high-temperature superconductor will turn into an insulator. S. Matsui et al. reported in J. Vac. Sci. Technol., B6 (3), 900 (1988) that an electronic circuit could be fabricated by applying a focused ion beam spot onto an oxide high-temperature superconductor so that the illuminated area would become an insulator in a width of no more than 1 .mu.m. However, as noted hereinabove, the thickness of the insulating layer must be controlled to less than 100 nm in order to fabricate a Josephson device of the tunnel junction type. In other words, it is very difficult to fabricate the desired tunnel junction type Josephson device by the FIB (focused ion beam) method.
On the other hand, it is possible to fabricate a Josephson device of a bridge type by the FIB method. As a matter of fact, A. E. White et al. reported in Appl. Phys. Lett., 53 (11), 1010 (1988) that the Josephson effect was observed in a bridge type sample prepared by the FIB method. However, no one has ever reported the case of successful fabrication of a tunnel junction type Josephson device by irradiation of an oxide high-temperature superconductor.